A snapped or shredded serpentine belt feels like the problem. Often it is only the symptom. On most vehicles one belt drives the AC compressor along with the alternator, and on many engines the water pump and power steering too. If anything on that belt seizes, the belt is what gives up. The AC compressor is one of the usual suspects.
How a seized compressor takes out the belt
The compressor pulley spins whenever the engine runs. On most systems an electromagnetic clutch locks the pulley to the compressor shaft only when the AC is on. That gives two ways for it to go wrong:
- The compressor seizes internally. The moment the clutch engages, the pulley stops dead and the belt drags across it, burning or tearing.
- The pulley bearing fails. The bearing inside the pulley wears out and locks up, or wobbles enough to walk the belt off. This one can happen with the AC switched off.
Some newer compressors have no clutch at all and are driven all the time, so a seized one stops the belt regardless of the AC switch.
Signs it was the compressor and not just an old belt
- A squeal or grinding that started or got worse when the AC turned on
- Black dust or rubber shavings around the compressor pulley
- The compressor pulley will not turn by hand, or it wobbles or grinds when spun (engine off)
- The belt was fairly new and still failed
- The AC had been blowing warm before the belt went
What happens when the belt is gone
With the belt off, the alternator stops charging, so the battery light comes on and the car runs on battery until it dies. If the belt also drives the water pump, the engine will overheat quickly. Steering gets heavy on hydraulic power steering. Do not keep driving it.
Why a new belt by itself may not fix it
Put a new belt on a seized compressor and it will fail again, sometimes within minutes. The cause has to be confirmed first. If it is only the pulley bearing or clutch, on some compressors that part can be replaced without opening the refrigerant system. If the compressor itself has seized, it is an internal failure, and that usually means metal debris has gone through the rest of the AC system. See AC compressor replacement for why that matters.
On some engines a shorter belt routed around the compressor can get a car driving again while the AC repair waits. Whether that is possible depends on the engine and belt layout.
How we handle it
We check the compressor pulley, the clutch and the rest of the belt-driven parts before fitting a belt, so you are not paying for the same belt twice. If the compressor has failed, we quote the full AC repair it needs, and you decide whether to do it now or get the car running first.